The use of optical frequencies to transmit intelligence-bearing-signals results in greatly increased information carrying capacity or bandwidth, and therefore greatly reduced transmission costs. This economic benefit has driven the development of optical communications over the past fifteen years. Various engineering advances continue to help realize the full bandwidth potential of optical communications systems.
One mode of optical communications which helps realize the bandwidth potential of such systems involves the frequency multiplexing of intelligence-bearing-signals in order to increase the realized bandwidth of the system. This frequency multiplexing is in many ways analogous to the frequency multiplexing which is well known in the field of radio transmission. In such multiplexing, known alternatively as frequency division multiplexing (FDM), or wavelength-division-multiplexing (WDM), different intelligence-bearing-signals are transmitted at different frequencies, thereby enhancing the information carrying capacity of the transmission system. See, for example, E. J. Backus et al, Electronics Letters, Vol. 22, No. 19, Sept. 11, 1986, where, in addition, heterodyne techniques are used to receive the signals. However, in such a frequency-division-multiplexed system, the values of the various frequencies, which carry the different information-bearing signals, must not drift, if they are to remain separated from each other and not interfere. In radio transmission, this is most economically accomplished by using a highly stable frequency generator, usually frequency stabilized by a quartz crystal resonator. However, in the optical domain, such stable frequency generators are prohibitively expensive. Consequently, other avenues must be sought to maintain the frequency spacing between the various intelligence-bearing-signals which are multiplexed in an optical communication system.
Techniques have been suggested in the prior art for frequency stabilizing single optical frequencies. From the perspective of the invention described in this patent, one of the most interesting of these techniques involves transmission through a Fabry-Perot resonator, or cavity, referred to here as a "Fabry-Perot". In such a system, a resonance line of the Fabry-Perot is used, in conjunction with a feedback circuit, to stabilize the frequency source, usually a laser. Exemplary of such stabilization techniques is the article by K. W. Cobb et al, Electronics Letters, Vol. 18, No. 8, page 336-337, Apr. 15, 1982, and Tot Okoshi et al, Electronics Letters, Vol. 16, No. 5, pages 179-181, Feb. 1980. A similar technique utilizing light reflected by a Fabry-Perot is reported by Sollberger et al in The Journal of Lightwave Technology, Vol. LT-5, No. 4, pages 485-491, Apr. 1987. However, the stabilization of a single frequency does not alleviate the problem of signal interference because signal interference still results due to the drift of the other, non-stabilized, frequencies. Additionally, even if all of the frequencies are stabilized by their own individual Fabry-Perots, the problem of signal interference still remains because of the drift associated with the Fabry-Perots themselves.
A technique for stabilizing a number of different optical frequencies has been suggested by D. J. Hunkin et al in Electronics Letters, Vol. 22, No. 7, pages 388-389, Mar. 27, 1986. In this technique, referred to as the "injection locking technique", a group of equally spaced frequencies is generated by frequency modulating a laser with an appropriate tone. The resultant equally spaced sidebands are used to stabilize a number of different optical frequencies. However, there is no simple feedback mechanism to effectively prevent the optical frequencies, which are to be stabilized, from "unlocking".